Somatotropic cells from three months old male mice, made hypoxic by either bleeding or exposure to 0.5 atm. barometric pressure in a decompression chamber, showed important ultrastructural changes when compared with normoxic controls, during the 36 hours that follow initiation of hypoxia. These changes were found to be similar for both types of hypoxic animals. An equivalent number of dark cells, cells with a swollen rough endoplasmic reticulum (RER), extremely granulated and degranulated, were successively--and in this order--seen along the experiment. Most of the observed differences between experimental and control animals appears as a magnification of the normal circadian rhythms. The degranulation rate, which probably reflects growth hormone (GH) secretion rate, was found to be about three times greater in hypoxic than in control mice. These findings indicate that a decrease in the red cell mass or its consequence (hypoxia), induce important ultrastructural variations in somatotropic (STH) cells, which may or not be specific.